Advanced high density data write strategy

ABSTRACT

A method of writing a mark to an optical disc includes receiving data to be written and generating a control signal for a laser pulse having a melt period that transitions to a growth period wherein the melt period is characterized by a melt power and the growth period is characterized by a growth power.

This is a continuation of U.S. Ser. No. 10/171,394, filed Jun. 12, 2002now U.S. Pat No. 7,301,883, which is hereby incorporated by referencefor all purposes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. patent application Ser. No.09/373,916, entitled HIGH DENSITY DATA WRITE STRATEGY, filed Aug. 12,1999, which is now U.S. Pat. No. 6,775,281, which is incorporated hereinby reference for all purposes; and U.S. patent application Ser. No.09/879,858, entitled HIGH DENSITY DATA WRITE STRATEGY, filed Jun. 12,2001, which is incorporated herein by reference for all purposes; andU.S. patent application Ser. No. 09/879,657, entitled HIGH DENSITY DATAWRITE STRATEGY, filed Jun. 12, 2001, which is incorporated herein byreference for all purposes.

FIELD OF THE INVENTION

The present invention relates generally to optical data storage. Morespecifically, write strategies for controlling the power of a writinglaser are disclosed.

BACKGROUND OF THE INVENTION

U.S. patent application Ser. No. 09/373,916, which was previouslyincorporated by reference, discloses various write strategies forwriting data to an optical discs. Those write strategies preciselymanipulate the state of regions of an optical disc to store data. Toincrease data density, it is desirable to develop more advanced writestrategies that even more precisely control regions of an optical discto store data.

Further, it is important to develop write strategies for a variety oftypes of optical discs, including phase change optical discs and dyebased optical discs. Such optical discs can be configured as rewritable,write once read many, read only or in any other appropriate manner. Inaddition such discs may conform to various standards defined for discsize, type of reading laser and thickness. Such standards include thevarious defined DVD standards, CD standards, and newer standards such asthe standards being developed for use with blue lasers. In each case,effective write strategies need to be developed for data densities to bemaximized.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIG. 1A is a cross section of a typical optical phase change disc 100.Optical phase change disc 100 includes a substrate 102.

FIG. 1B illustrates an example of a typical dye-based CD-R type media.

FIGS. 2A and 2B show other embodiments of pulse width modulated writestrategies.

FIG. 2C is a diagram that illustrates a write strategy that uses a pulsehaving a variable power.

FIG. 2D is a diagram that illustrates a write strategy that uses a pulsehaving a variable power.

FIG. 2E illustrates a write strategy where the melt period durationT_(m) is controlled in addition to the location of the transition fromgrowth power P_(g) back to read power P_(r).

FIG. 2F illustrates a write strategy where a premelt period at thegrowth power precedes the melt period and the growth period.

FIG. 2G is a diagram of a write strategy that includes a premelt periodthat has a variable premelt power and a variable transition time to meltpower.

FIG. 2H illustrates a write strategy that uses a progressivelyincreasing power over three periods.

FIG. 2I shows a write strategy used in a multilevel system that isphysically compatible with a DVD-RAM system and media.

FIG. 2J illustrates a write strategy used to create multilevel marks.The strategy may be used, for example, DVD-RAM media, and has thebenefit of reliable disc over-write (DOW) performance.

DETAILED DESCRIPTION

It should be appreciated that the present invention can be implementedin numerous ways, including as a process, an apparatus, a system, or acomputer readable medium such as a computer readable storage medium or acomputer network wherein program instructions are sent over optical orelectronic communication links. It should be noted that the order of thesteps of disclosed processes may be altered within the scope of theinvention.

A detailed description of one or more preferred embodiments of theinvention are provided below along with accompanying figures thatillustrate by way of example the principles of the invention. While theinvention is described in connection with such embodiments, it should beunderstood that the invention is not limited to any embodiment. On thecontrary, the scope of the invention is limited only by the appendedclaims and the invention encompasses numerous alternatives,modifications and equivalents. For the purpose of example, numerousspecific details are set forth in the following description in order toprovide a thorough understanding of the present invention. The presentinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the present invention is notunnecessarily obscured.

Information may be stored on an optical disc by creating regions or“marks” having a different reflectivity than the surrounding surface ofthe disc. In an optical phase change disc, such regions are formed byirradiating the surface of the disc with a writing laser that causes aregion to be warmed and melted or partially melted. As the region cools,the region may change to a crystalline or amorphous state or somecombination of crystalline and amorphous states. When a reading laser isincident on such a region, the reflected light can be measured and thestate of the region can be determined. The state of the regionrepresents stored data. Different levels of reflectivity may representdifferent data levels.

FIG. 1A is a cross section of a typical optical phase change disc 100.Optical phase change disc 100 includes a substrate 102. A firstdielectric layer 104 is deposited on the substrate. A recording layer106, which is composed of a phase change material, is deposited on topof dielectric layer 104. A second dielectric layer 108 is deposited overrecording layer 106. A reflective layer 110 is deposited over dielectriclayer 108. Finally, a protective resin layer 112 is deposited overreflective layer 110. The layers described above are provided as anexample only and that the techniques described herein are applicable toother types of phase change discs as well as other optical discs thatutilize different recording mechanisms. U.S. Pat. Nos. 5,136,573 and5,144,615, (the '573 and '615 patents, respectively) issued toKobayashi, which are herein incorporated by reference for all purposes,describe techniques for multilevel recording using optical phase changediscs. Kobayashi describes lowering the reflectivity of an initiallyamorphous region by forming crystalline material and that morecrystalline material may be formed as the writing laser power isincreased. Kobayashi also discloses an overwrite technique that forms acrystalline region using a low level biasing energy and increases thereflectivity by forming a crystalline region at certain spots usingenergy pulses that create amorphous regions.

FIG. 1B illustrates an example of a typical dye-based CD-R type media.CD-R discs such as that shown in FIG. 1F typically include a layer ofmaterial comprising a light sensitive organic dye 160, deposited on apolycarbonate substrate 164, and coated by a metal reflective layer 166,and a protective layer 168. The metal reflective layer 166 is typicallyof Gold or Aluminum; the choice of the metal, or alloys of such metals,is a preference of the media manufacturer. Additionally, variations ofthe layer structure are common; with the addition of other layersbetween the dye 160 and the reflector 166 layers and/or between the dye160 and the substrate 164; for optimizing thermal and/or opticalcharacteristics of the disc as demands on the media advance towardsfaster writing and reading and for increased storage capacity.

Dye such as that which comprises layer 160, changes chemical state whenexposed to a write laser. The reflectivity of the dye layer 160 is thenread with a read laser having a lower power which does not furtherchange the state of the dye. Suitable dyes generally include organiccompounds with conjugated double bonds, and include compounds in thecyanine, squarylium, and azomethine families. Typical, commerciallyavailable dye based media are the most frequently used, but otherspecific types such as Phthalocyanine may be additionally used. Withconventional dye based discs, the mark reflectivity toggles between twovalues in a binary data encoding scheme. The physical length of the markis typically 0.83 μm for CD-R media and 0.4 μm for DVD-R media.

Other forms of optical discs use materials that are designed to allowmultiple write cycles. One such disc uses a layer of material thatundergoes a reversible phase change when heated by a write laser. In anoptical phase change disc, such regions are formed by irradiating thesurface of the disc with a writing laser that causes a region to bewarmed and melted or partially melted. As the region cools, the regionmay change to a crystalline or amorphous state or some combination ofcrystalline and amorphous states. When a reading laser is incident onsuch a region, the reflected light can be measured and the state of theregion can be determined. The state of the region represents storeddata. Different levels of reflectivity may represent different datalevels.

Phase change materials that do not support reversible characteristicshave also been employed for Write Once Media. The techniques disclosedrelating to write once dye based media are also applicable to phasechange materials based write once media.

FIGS. 2A and 2B show other embodiments of pulse width modulated writestrategies. In FIG. 2A, the duty cycle of the pulse is varied over aperiod τ. The power level is maintained at the read level, P, until itis increased to a write level P_(w) for a duration τ_(on), which is apercentage of the period τ. While writing to a media using thesestrategies, several different duty cycles may be used to write desiredmarks to the track. With dye based media, the different pulse widthscontrol the amount of dye bleaching in the recording layer so as toproduce multiple levels of reflectivity. In this embodiment, the levelP_(w) may be held constant while the pulse width duration τ_(on)changes. In the embodiment of FIG. 2B, a bias power P_(b) is added tothe pulse train to enable the writing of multi-level marks on DVD-Rmedia at a higher speeds including, for example, a 14.4 m/sec trackvelocity. The power level is first increased from P_(r) to P_(b) for atime τ_(b) until the beginning of a period τ when the power level isincreased to P_(w) for a time τ_(on). The duty cycle over τ is variedaccording to the write strategy shown in FIG. 5B, and as describedabove. After a time τ_(on), the power is decreased to P_(b) for theremainder of τ. In this strategy, the actual powers P_(b) and P_(w) andthe durations of τ and τ_(on) will depend on the disc, write speed, andmark size desired. These pulse width modulated methods have been foundto be useful with conventional dye-based write once optical recordingmedia, often referred to as CD-R, and DVD-R. They may also be used withwrite-once phase change media.

FIG. 2C is a diagram that illustrates a write strategy that uses a pulsehaving a variable power. P-on is varied so that the shape of a markwithin a cell is varied. It should be noted that the term cell or datacell as used herein is used simply to refer to a region in which a markis written and is not meant to imply that any a defined cell boundarymust necessarily exist or that cells or regions with marks or marksthemselves do not overlap. In some embodiments, specific data regionsare defined and the position of a mark within the region is controlled.An example of such a strategy is illustrated in FIG. 2C and theresulting marks are shown in FIG. 7B. The pulses are defined by fiveparameters: Tau, Tau_g, Tau_m, P-g and P-m. In the example shown, thepulse begins with maximum power P-m and continues for a duration Tau_mat maximum power. Then, the pulse transitions to an intermediate powerlevel P-g for a time Tau_g. After interval Tau_g, the pulse ends. Aswith all of the described write strategies, when the pulse ends, thepower may either be zero power or a biasing power. Varying the powerduring a pulse changes the shape of a mark written by the pulse. In oneembodiment, lowering the power in the middle of a pulse decreases thesize of the amorphous mark by promoting the growth of crystallinematerial beginning at the outside boundary of the mark. As is discussedin detail below, marks of different shapes and sizes can be written toproduce different reflectivity level signals. This can be accomplishedby a suitable choice of Tau_m, Tau_g, Pm and Pg for each level ofreflectivity desired.

FIG. 2D is a diagram that illustrates a write strategy that uses a pulsehaving a variable power. This strategy is useful for writing to DVD-RWmedia at, for example, 3.5 m/s or 7 m/s track linear-velocity. P_(g) maybe varied so that the shape of a mark within a cell is varied. Thepulses are defined by several parameters shown and defined in FIG. 5F:Tau, T, P, T, T_(F), T_(r), P, and P. In the example shown, the pulsebegins with a bias power P_(r) which continues until the beginning ofthe duty cycle. At t=0, the pulse transitions to a melt power P_(m) andmaintains this level for T_(m) which in this embodiment is equal toT_(F). Then, the pulse transitions to an intermediate growth power levelP_(g) for a time T. After an interval of t=T_(m)+T_(g), the pulse ends,transitioning back to the P_(r) level for the remainder of the periodTau.

The technique of FIG. 2D enables the formation of multi-level marks withDVD-RW, and DVD+RW media, thus enhancing the storage capacity, andperformance of these media. The multi-level response from the media isobtained by varying the duration of T. The melt-power (P_(m)) in thisembodiment preferably ranges from 12 to 14 mW, and is immediatelyfollowed by a growth power (P_(g)) preferably ranging from about 6 to 9mW. These parameters will depend on the media used, and the speed atwhich they are employed. This multi-level write strategy can be used tomake marks in the range of 0.2 μm within a data cell size which may befrom 0.3 to 0.4 μm. The bias power P_(r) allows for direct overwritingon written tracks, thus making the strategy useful for DVD+RW, andDVD-RW media types.

Variations may be employed and used for multi-level writing on a numberof media types having different characteristics. Multiple levels ofreflectivity can be obtained by making marks of different shapes andsizes, by suitable choice of Tau-m, Tau-g, Pm and Pg for the differentlevels. The dynamics of controlled mark formation depend on the pulsedurations, powers and track velocity, besides media thermalcharacteristics such as conductivity, specific heat, and cooling rate.Higher speeds typically require higher powers to write marks.

FIG. 2E illustrates a write strategy where the melt period durationT_(m) is controlled in addition to the location of the transition fromgrowth power P_(g) back to read power P. In this embodiment, for eachdesired level, the location of the transitions from P_(m) to P_(g) andthe location of the transition from P_(g) to P_(r) are varied for eachlevel, and optimized to maximize dynamic range and minimize noise. Inone advantageous embodiment, pulses with a longer growth period T_(g)have a shorter melt period T. This may be controlled by the parameterT_(v), which may vary between 0 and 1. In some embodiments, T_(v) isoptimized globally, and is the same for all levels. It is also possibleto have T_(v) vary for different levels, which provides totallyindependent control and optimization of both T_(g) and T_(m) at alllevels. As is demonstrated by the examples below, this techniqueproduces well resolved levels with a low “SDR”, wherein SDR for eachlevel is defined as sigma (the standard deviation in reflectivity valueacross a large number of same level marks) divided by the dynamic range(the reflectivity distance between the most reflective and leastreflective levels).

The strategies embodied in FIGS. 2D and 2E allow controlled formation ofa crystalline region. The disc material is first melted by the meltpower and then cooling is slowed by the growth power to allow formationof a crystalline region. As the melt power period is increased, theamount of crystalline region increases. When the growth power is turnedoff at the end of the growth period, the disc material cools morerapidly and an amorphous region forms. Thus, the timing and period ofthe growth period controls the reflectivity of the disc.

In addition to varying the timing of the transition from melt power togrowth power and the length of the growth period, the entire waveformincluding the melt period and the growth period may be shifted in timein relation to previous and future pulses. The shift may be a functionof the mark being written or the shift may be a function of a group ofadjacent marks.

FIG. 2F illustrates a write strategy where a premelt period at thegrowth power precedes the melt period and the growth period. Thebeginning of the transition from P_(m) to P_(g) is fixed at t=T_(F), andthe location of the leading melt pulse transition from P_(g) to P_(m) isaltered in combination with the location of the transition from P_(g)back to P_(r) to produce the multi-level marks. One advantage ofincluding a premelt period is that the material to be melted is heatedin preparation for melting, but the lower power delivered during thepremelt period does not interfere with the cooling of adjacent materialwhere a previous mark was written. If full melt power is applied at thestart of a data cell, heat from the cell tends to conduct to adjacentcell and interfere with the write process for an adjacent mark. In theembodiment shown, the timing of the transition to the melt period isvaried according to the mark being written. In other embodiments, thetransition to the melt period is fixed.

FIG. 2G is a diagram of a write strategy that includes a premelt periodthat has a variable premelt power and a variable transition time to meltpower. This strategy is especially useful because controlling thepremelt power as well as the time transition between the premelt powerand the melt power further enables interference with the formation of aprevious mark to be avoided while still achieving the required heatingof the disc material for formation of the current mark. In theembodiment shown, a delay of period T_(F)−T_(d) is imposed prior to thestart of the melt pulse. The premelt power applied during the premeltperiod helps to erase prewritten marks and improve overwrite performancewithout interfering with adjacent mark formation. The period T_(d) andthe power level during the delay period T_(F)−T_(d) are both chosen tooptimize performance for specific media. The premelt period may bedetermined empirically. In one embodiment, the period length and powerlevels are calibrated based on a signal read from the disc. As isillustrated by arrows 510, the premelt power, the timing of the premeltto melt transition, and the timing of the ending of the growth periodare all adjusted according to the mark or group of marks being written.In certain embodiments, one or more of these parameters may be fixed asappropriate for a given system.

FIG. 2H illustrates a write strategy that uses a progressivelyincreasing power over three periods. The timing of the transitionsbetween the period is varied according to the mark that is beingwritten. The placement within the full mark period, Tau, of transitionsfrom P_(r) to P_(g) and from P_(g) to P_(m) are controlled to producethe best SDR over all levels. It should be noted that while thevariables P_(r), P_(g) and P_(m) are still used to denote power levels,those power levels do not necessarily correspond to read, growth andmelt powers. For example, this write strategy is applied to write oncedie based or phase change media where the power delivered tends todarken more of the media and the process of full melting and partialrecrystallization is not relevant.

FIG. 2I shows a write strategy used in a multilevel system that isphysically compatible with a DVD-RAM system and media. A bias powerP_(b) is added to a train of pulses to enable the writing of multi-levelmarks on DVD-RAM media. The power level is first increased from P_(r) toP_(b) for a time τ_(b) until the beginning of a period τ when the powerlevel is increased to P_(w) for a time τ_(on). The duty cycle over τ isvaried to deliver a variable amount of power. After a time τ_(on), thepower is decreased to P_(b) for the remaining period τ. In thisstrategy, the actual powers P_(b) and P_(w) and the durations of τ andτ_(on) will depend on the disc, write speed, and mark size desired. Thisstrategy is useful for writing to DVD-RAM media at, for example, 3.84m/s track linear-velocity. The technique of FIG. 5K enables the creationof multi-level marks with DVD-RAM media, thus enhancing the storagecapacity, and performance of these media. The multi-level response fromthe media is obtained by varying τ_(on).

FIG. 2J illustrates a write strategy used to create multilevel marks.The strategy may be used, for example, DVD-RAM media, and has thebenefit of reliable disc over-write (DOW) performance. The strategy ischaracterized by seven parameters: τ, τ_(on), P, P, P_(b), τ_(b) and P.In the example shown, the pulse begins at P. At the beginning of periodτ, the power of the pulse is increased to P_(b), for a duration τ_(b),at the end of which time the power is increased to P. The power of thepulse is held at P_(m) for a duration τ_(on), after which the power isdropped to the P_(r) level. The combined period at P_(m) and P_(b)comprise a melt period. A subsequent series of pulses is then formedwith a short duty cycle, and power alternating from P_(w) to P; thisseries of pulses continues for the remaining time in τ, at the end ofwhich the power level is returned to P_(b).

In the strategy of FIG. 2J, τ determines the length of the data cell inwhich the marks are written. The warm up duration at the chosen biaspower P_(b) combined with the melt pulse ensure that previously writtenmarks within the data cell, and on the media, are erased completelyduring the writing of the new mark. The melt pulse creates a moltenregion of the media which upon quenching leaves an amorphous mark. Thisprocess ensures reliable direct over writing during amorphous markformation for multi-level recording. Amorphous marks can be writteninside the data cell during the multi-pulse train which continues toheat the material. The size of the amorphous mark, and therefore thelevel of reflectivity of the region, is determined by the pulsedurations, power levels, and number of pulses within the multi-pulsetrain. In the embodiment shown, there are two power steps before thepulse train, the warm up at bias power and the melt pulse. In otherembodiments, additional power levels are used in additional steps areintroduced before the pulse train. These steps comprise subperiods ofthe melt period.

Advanced data write strategies have been disclosed. The strategies maybe applied to control the writing laser in any appropriate optical discsystem. Data to be written to a disc is input to a write strategyprocessor that generates a laser control signal that causes the writinglaser to output the appropriate pulses according to the write strategy.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing both the process and apparatus of the present invention.Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein, but may be modified within the scope andequivalents of the appended claims.

1. A method of writing a mark to an optical disc comprising: receivingdata to be written; generating a control parameter having a value withina range from about zero to about one; and generating a control signalfor a laser pulse having a predefined period comprising a melt periodand a growth period, the melt period comprising a transition between amelt power configured to melt a region of the optical disc and a growthpower configured to allow formation of crystalline material, the growthperiod including a transition from the growth power to a read power,wherein a duty cycle of the melt period and a duty cycle of the growthperiod are adjusted with respect to each other according to said controlparameter.
 2. The method according to claim 1, wherein the duration ofthe growth power causes the formation of crystalline material.
 3. Themethod according to claim 1, wherein a position of the transitionbetween the melt power and the growth power within the melt period isvaried based on the data to be written.
 4. The method according to claim1, wherein a position of the transition between the melt power to thegrowth power within the melt period is varied according to the markbeing written.
 5. The method according to claim 1, wherein a position ofthe transition between the melt power and the growth power is variedaccording to a previously written mark.
 6. The method according to claim1, wherein a position of the transition between the melt power and thegrowth power is fixed within the predefined period of the laser pulse.7. The method according to claim 1, wherein the control signal for thelaser pulse further comprises a delay period comprising a premelt power,wherein said premelt power is varied according to the mark beingwritten.
 8. The method according to claim 7, wherein the premelt poweris varied according to a previously written mark.
 9. The methodaccording to claim 7, further comprising the step of: calibrating aduration of the delay period based on a signal read from said opticaldisc.
 10. The method according to claim 7, further comprising the stepof: calibrating a level of the premelt power based on a signal read fromsaid optical disc.
 11. The method according to claim 1, wherein aposition of the transition from the growth power to the read power isshifted in time with respect to a boundary of the predefined period ofthe laser pulse according to the mark being written.
 12. The methodaccording to claim 1, further comprising varying said value of saidcontrol parameter within said range according to a level of reflectivityto be obtained for said mark.
 13. A write strategy control signalgenerator comprising: an input for receiving data to be written to anoptical disc; and a processor configured to generate (i) a controlparameter having a value within the range from about zero to about oneand (ii) a control signal to control a laser pulse having a predefinedperiod comprising a melt period and a growth period, the melt periodcomprising a transition between a melt power configured to melt a regionof the optical disc and a growth power configured to allow formation ofcrystalline material, the growth period including a transition from thegrowth power to a read power, wherein a duty cycle of the melt periodand a duty cycle of the growth period are adjusted with respect to eachother according to said control parameter.
 14. The write strategycontrol signal generator according to claim 13, wherein said processoris further configured to generate said control signal having a delayperiod, the delay period comprising a premelt power, wherein saidpremelt power can be varied above and below said growth power.
 15. Thewrite strategy control signal generator according to claim 14, whereinsaid processor is further configured to calibrate a duration of saiddelay period based on a signal read from said optical disc.
 16. Thewrite strategy control signal generator according to claim 14, whereinsaid processor is further configured to calibrate a level of saidpremelt power based on a signal read from said optical disc.
 17. Thewrite strategy control signal generator according to claim 14, whereinsaid processor is further configured to adjust a level of the premeltpower, the timing of the transition between the growth power and themelt power and the timing of the transition between the growth power andthe read power based on a group of marks being written.
 18. The writestrategy control signal generator according to claim 14, wherein saidprocessor is further configured to select a premelt power level and aduration of said growth period based on media of said optical disc. 19.The write strategy control signal generator according to claim 14,wherein said processor is further configured to determine a level ofsaid premelt power and a duration of the melt power based on a signalread from said optical disc.